I think about that one a lot. It goes all the way back to the CMB, which is so "big" that it is literally everywhere you look and the shapes we see were apparently at the quantum scale.
According to current theory AIUI, cosmic inflation greatly influenced the CMB. It ended approximately 10^-32 seconds after the Big Bang:
"Cosmic inflation is believed to have occurred in an incredibly brief, rapid, and exponential expansion phase lasting from approximately 10^-37 to 10^-32 seconds after the Big Bang. During this minute interval, the universe expanded by a factor of at least 10^26, and potentially as much as 10^50."
"Cosmic inflation is believed to have occurred in an incredibly brief, rapid, and exponential expansion phase lasting from approximately 10^-37 to 10^-32 seconds after the Big Bang. During this minute interval, the universe expanded by a factor of at least 10^26, and potentially as much as 10^50."
Quite a theory, cosmic inflation...